(a) Field of the Invention
The present invention relates to a packet transmission method and device for cable network communication. Particularly, the present invention relates to a system packet transmission method and device according to the data-over-cable service interface specifications (DOCSIS) for the cable network communication.
This work was supported by the IT R&D program of MIC/IITA [2006-S-019-02, The Development of Digital Cable Transmission and Receive System for 1 Gbps Downstream].
(b) Description of the Related Art
In a cable modem termination system (CMTS) of a cable head-end, a sync message is transmitted through a media access control (MAC) layer and a physical media dependent (PMD) layer.
The sync message is generated by the MAC layer and is then transmitted through the PMD layer. In this instance, timing synchronization between the two layers must be accurate. However, in general, it is impossible to share system clock signals since the system clock rates used by the two layers do not correspond with each other. In detail, even when the MAC layer uses the local clock signals thereof to transmit data to the PMD layer with an accurate output rate, the local clock signals do not match the local clock signals of the PMD layer, and no accurate synchronization is maintained. The final output of the MAC layer has the transmission rate of 26.97032 Mbps (megabits-per-second) or 38.81070 Mbps with the 188-byte MPEG-2 (Moving Picture Experts Group-2) transport stream packets. The PMD layer outputs RF signals of 5.056941 Msps (mega symbols-per-second) or 5.360537 Msps after passing through forward error correction (FEC) and 64 or 256 QAM modulation.
As described above, the MAC layer and the PMD layer have different outputs and accordingly require different system clock rates. However, it is not easy to generate two system clock signals by using a shared clock signal unit. The MAC layer generates a sync message and transmits it to the PMD layer, but timing jitter of the sync message exists because of the different output rates of the MAC layer and the PMD layer. Therefore, in order to prevent this, accurate timing synchronization between the two layers is required. For this purpose, a very precise PLL device is needed, and the MAC layer's processing becomes very complicated.
The MAC layer generally processes data by the burst mode. That is, the MAC layer processes the data only when receiving broadband data from the backbone network. Further, the PMD layer processes the data by the continuous mode. Therefore, the PMD layer needs to insert a null packet into the output data of the MAC layer so as to maintain the output rate, which further hinders timing synchronization of the two layers.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.